1. Field of the Invention
This invention pertains to a tandem spring and service brake actuator for a pneumatic braking system. More specifically, the invention is directed to integrating a biased double check valve into a tandem actuator.
2. Discussion of the Art
Conventional pneumatic braking systems for heavy duty vehicles such as trucks, buses, and the like use tandem brake actuators to effect actuation of the vehicle foundation brakes. The tandem brake actuators include a service brake actuator that is responsive to air pressure. The service brake actuator effects a service brake application by receiving a pneumatic signal, for example as provided by an operator depressing a foot pedal a preselected amount. The tandem brake actuator also includes a spring brake actuator to effect emergency and/or parking brake application. A heavy duty spring is associated with the spring actuator and urges an actuator rod in a brake application direction. Pneumatic pressure opposes the spring force during normal operation to release the emergency/parking brake so that service brake application can be effected as desired. The pneumatic pressure is vented, however, during emergency conditions or when the vehicle is parked so that the spring force can apply the emergency or parking brake.
As indicated above, both the service brake pressure and the hold off pressure of the spring actuator are controlled by pneumatic signals from the vehicle operator. Separate pneumatic lines effect control of the vehicle service and parking/emergency braking systems. It is contemplated further that electronic braking systems or electro-pneumatic braking systems form a part of the conventional pneumatic braking systems for heavy duty vehicles.
In present systems, a supply line extends from a dash valve and upon actuation supplies air pressure to effect emergency or parking brake application. A double check valve is associated with an anti-compounding circuit. The delivery from a service relay valve assures that the operator demand for pressure application, if no pressure is already present, is obtained from the relay valve for application to the spring brake. If the dash valve is actuated and the operator is unaware that supply pressure is already being delivered, i.e., the brakes are already actuated, the double check valve assures that the brake chamber receives the desired pneumatic signal. Compounding of brake forces is eliminated with the double check valve since supply pressure to the parking or emergency brake and the service pressure are not applied simultaneously through use of the double check valve. If the spring side of the tandem brake actuator is not pressurized and pressure is provided to the service side, a cumulative brake effect could occur--an undesired result.
Roll-back is associated with the transition from service braking to park/emergency braking. For example, if a truck is parked on an uphill incline, and the operator depresses the brake pedal, the truck is held in place. The operator then actuates the park brake, for example pulling a dash valve. As the operator lifts the foot from the brake pedal, there is a transition from where the service brake releases and the spring brake is actually applied. In this transition, there is the potential that the truck will roll backward until the spring brake is fully actuated. Consequently, brake system design must also address these issues and provide an anti-roll-back feature.